I’m not keeping up with all my email too easily here in the UK — I’ve been on the road quite a bit — but this one I have to jump on right away. A team of researchers in Italy are claiming to have directly detected dark matter particles.

Short background: we know dark matter exists. We also know it must be made up of particles that are very difficult to detect (or else we’d see them, of course). A leading contender are WIMPs, or weakly interacting massive particles, specifically a weird particle called an axion. We still have not directly detected and identified an axion in any particle experiments, though LHC (and more on that later, of course!) may bag one soon.

DM particles form a cloud around and throughout the Milky Way. Although they don’t interact with normal matter terribly well (hence the weakly interacting part of their name) they do sometimes slam into normal matter. You can build a detector to look for that interaction — it would make little flashes of light — but again you need to look very carefully.

The Italian experiment looked very carefully. One of the things they looked for was a modulation in the signal, a change over time. They wanted to see the number of flashes of light from DM hitting normal matter go up and down by a few percent, with a maximum in June and a minimum number in December. Why?

Imagine you are in a car, driving through a cloud of bugs. If you hold your hand out the window, a bunch of bugs will hit it (ewwww). Now if you throw a ball out the window into the direction of the car’s motion, it will hit more bugs, because it’s moving into the cloud faster. If you throw a ball behind you, then the ball will be moving slower relative to the bugs, and fewer bugs will hit the ball.

A similar thing is happening with the Earth. The whole solar system is orbiting the center of the Milky Way at about 250 kilometers per second. But the Earth is also orbiting the Sun. When the Earth is at one part of its orbit, its velocity (30 kps) adds to that of the solar system, but six months later it’s headed the other way, and its velocity subtracts.

If the Earth is slamming into dark matter particles, then we should hit more when the Earth and solar system velocities are in the same direction, and hit fewer when the Earth is moving in the opposite direction of the solar system as a whole six months later. So not only should we see the number of hits go up and down every six months, but that oscillation must line up with the correct dates (June for the former, and December for the latter).

That is precisely what the Italian team found. Here are their results:

This is a little weird, so let me explain! The horizontal axis is time, measured in days. For the first 2700 days or so, they used an experiment that could detect the flashes, but after about 3200 days switched to an experiment that was a lot more sensitive. So you can see the data points (the gray circles) in the first half of the graph are a little ratty, but in the second half are much smoother. They then fit a smooth sine wave to the data points, to get the solid wavy line. The dashed lines are where the signal is at its max, and the dotted lines where it’s at its min. The number of particles detected goes up and down once per year, as predicted, and the max is also on June 2, right when the Earth is "facing into the wind", so to speak.

Holy Haleakala.

I have to say, this is an incredible graph, and an incredible result. It needs to be confirmed independently, but it’s already pretty convincing! The modulation of the signal is very strong (they are confident of it to the 99.99% level) and the phase — its alignment in time — is right on the money.

Now, this may not mean it’s dark matter. It is certainly something interstellar extraterrestrial, though! The phase and alignment are too perfect to be anything else, I think. But dark matter is a very likely candidate.

I’ll need to see what others in the field think of this (I happen to have Brian Cox handy, but I’ll have to ask him later about this since he’s busy). But it looks really solid. If true, the next step is to use even more sensitive detectors, and ones that can distinguish between different types of particles. In the meantime, as other detectors come online (like one at LHC meant to look specifically for axions), we are heading ever closer to solving one of the biggest mysteries in astronomy, if not in all of science. Dark matter makes up 23% of the stuff of the Universe, and 80% of all matter is dark matter. What the heck is it?

Soon enough, we may know.

Update: Dagnappit, I should read the other science blogs before writing mine! Cosmic Variance has this story as well, and is also skeptical of the result being dark matter, and also agrees that the modulation is real and probably something interesting. However, he’s a bit more forceful about it than I am.

Beware! Pareidolia is everywhere and you and the Italians may be expericencing it now. When you want to see something badly enough it may pop out of the most unlikly and arcane of sources. Or it may be visible because the data were constructed to produce the “picture.”

Excellent job, as usual, of making this data comprehensible for the layman, Phil.

But now you’ve got be excited and more than a little intrigued… I’m curious… what kind of detector was used to attain the data points and what, exactly, was detected? You mention that interaction between normal matter and dark matter “WIMPS” produce “little flashes of light”. What properties do the “flashes of light” contain that make them distinguishable as a result of the normal matter / dark matter interaction?

Funny, I saw this one coming, and had the story last week! Same conclusion as you and cosmic variance, though, that this is most likely not dark matter (at least, isn’t overwhelming evidence for it), but that the signal is something new!

If dark matter interacts primarily by gravity, should not the gravity well of the Earth/Moon system have within it some amount of attracted dark matter that, perhaps, can be detected by its gravitational effect?

It would seem that if a galaxy can attract a huge cloud of dark matter (or a huge cloud of dark matter attracts a galaxy) that little centres of mass should have smaller clouds of dark matter around them, and that it should be detectable in the usual ways we might detect gravitational effects.

If not the Earth/Moon, then at least the solar system which has a bigger mass in the middle.

For instance, the mass of the dark matter floating aroudn the solar system should affect the orbits of everything in the solar system.

Something else comes to mind. At the relative speed difference in our summer/winter passage through space – which is small on a cosmological sense – the difference in interaction with WIMPS would be extremely low. Consdiering how very little interaction on a given volumn of space is taking place (prima-facia, we have not found anything yet as connected with interaction, and only thought to exist on galictic scales) the likelihood of DM being found so easily seems improbable. We might have a better chance of detecting a fart in a windstorm. Wait-and-see, and demand several confirming data sets from other experiments by other investigators. If it turns out that DM is so easily detected and interacting at such high levels, we then have to look at many other interactions and account for DM’s presents. This could easily cause a rewriting of our sciences from how much effect it has on the internal structure of stars to why women’s hose runs so easily.

Is the implication here that this phenomenon is static with respect to some external frame of reference while pretty much everything we can observe in the galaxy is moving through it? This seems like an impossible symmetry to me. It reads very much like a positive result to the Michelson-Morley etherwind experiment.

Given the relative locations of their local minima and maxima, I would be very concerned that there might be some sort of earth seasonal effect playing some impact on their data. They seem to do a little controlling for temperature, but you don’t have to adjust the phase of that cycle by very much to have the minima and maxima correspond to a solstice. I would be extremely interested to see this experiment reproduced in Australia or South Africa (or better yet, in space, away from the Earth). Of course, I don’t have so much time to read their work in detail, so maybe I’m missing something.

Something else comes to mind. At the relative speed difference in our summer/winter passage through space – which is small on a cosmological sense – the difference in interaction with WIMPS would be extremely low.

“Extremely low” is a subjective term. You’re right that the speed difference is small on a relativistic scale cosmologically, but I don’t think it renders the data observed in this instance statistically invalid. It may not be Darm Matter, but the data does seem to indicate SOMETHING of relevance.

Ethan: Hi! Didn’t know you read Phil’s page. I haven’t forgotten about the halo figure rotation stuff, just been very busy with other projects… .

Squid: While that is in principle true, galactic dark matter is too “dynamically hot”(1) to cluster around small bodies. What that means is that the typical velocity difference between pairs of adjacent dark matter particles is too high (probably around 200 km/s), so that even if you gathered a bunch of them in one spot, they’d all fly off in different directions unless you manage to get enough mass(2) of them that their gravity can hold onto them despite how fast they’re moving. The earth and moon don’t have enough mass to do that.

Viggen: Yes, it is fixed with respect to the galaxy as a whole… because it *is* the galaxy as a whole (at least, most of the mass of it). It’s not a universally special frame (ie. it would be different if we were in a different galaxy moving with respect to ours), but it is a well-defined frame with respect to us.

I didn’t get the part about June and December. In June the Earth’s velocity vector is more or less aligned parallel with the Sun’s velocity vector about the galactic center, and in December the Earth’s velocity vector is antiparallel to the sun’s. Is that right?

If so this begs the question about why dark matter would essentially be stationary with respect to the Milky Way galaxy. If it has such little interaction with baryonic matter there is no particular reason that its motion would correlate well with a big lump of baryonic matter (i.e., a galaxy). I guess that’s why I’m skeptical particularly when the dates given correlate very well with the seasons which could indicate instrumental issues.

alfaniner–thank you very much for the Thrice Upon a Time reference! i read it in the 80’s, so it has been a few years…i didn’t remember the name and only vaguely remembered the plot. i was reminded of it when i started reading the hysteria about the LHC coming online and creating strangelets and black holes and lions and tigers and bears, oh my!

sean carroll–good point. it would suck if it turned out to be electrical noise from the air conditioners that come on more often during the warmer months or extra vibration from the tour bus groups that drive by during june.

I read the blog (actually a guest blog by Jaun Collar) and I thought he was more than a little bit more forceful in his skepticism. More like the scientific reviewer version of a terminator. I came away with the idea he thought it was not a big deal. In fact he trashed DAMA on saying they have proof of dark matter.

“Occam is turning in his grave, rusty razor still in hand…….I try to teach my students that a good experimentalist does not need any critics: he or she is his/her own worst enemy. If you don’t feel a sincere drive to debunk, test and revise your own conclusions, you should be doing something else for a living. This intent is seemingly absent from the DAMA collaboration.”……looks like Juan Collar is a firm Saganist!

Well, that’s the nature of science. You have a hypothisis, you test, and you get the result you want, or you find something totally different. Wash, rinse, repeat.

Sometimes, as with the Cold Fusion flap, scientists put the cart before the horse and are sorry for doing so later. But it sounds like in this case, that there is a very real SOMETHING that they’ve detected. It might be DM, (great!) or something we didn’t know about before (even better, IMHO!) that is causing this signal modulation.

Sean Carroll: give the poor guy a break. He’s on jet lag, has his head full of european stuff, sleeps at Giagia’s place and above all, has to borrow someone else’s computer just to access his blog. I’m sure he’ll soon enough post a more precise take on that particular news.

There is no reason to assume that whatever is being detected is stationary to the Milky Way. For example: the solar wind is moving at a substantial rate from the sun, but a sensitive detector on the leading face of the Moon (since one on the Earth would have interference from its magnetic fields and atmosphere) in its orbit around the Earth should find a variance in the rate of solar wind particles that would have a maximum at the time of the full moon and a minimum at the time of the new moon. This does not require the solar wind to be static relative to the sun. Whatever the Italian researchers have detected, its general motion around the galaxy can be more-or-less random and chaotic, but the detector passing through this would have a variance in the number of detections based on the detector’s motion, not that of what is being detected.

While your point of seasonal weather variance is important, the nature of such scintillators require that they be placed deep underground to minimize the effects of cosmic rays. This also helps reduce the local weather effects.

So I’m sitting here being all “layperson” and stuff and I realize that my bobble-headed nodding was halted by a self-introduced bit of irony.

I detected a bit of self-loathing in Phil’s post in the sense of “woo woo!” and then “oh, shoit!”

As a sideliner, I’m cheering both sides of the “team” since their our interactions fascinates me to no end.

I’m rooting for DM being something that has absolutely no physically detectable connection with this universe other than gravity. If it doesn’t turn out to be the gravitational interaction of baryon-like particles across branes, I will be sorely disappointed.

On the plus side, you guys are doing a fantastic job educating science hooligans like me.

If so this begs the question about why dark matter would essentially be stationary with respect to the Milky Way galaxy.

Nothing implies that whatever-it-is needs to be “stationary with respect to the Milky Way”.
When the Earth is “facing into the wind”, it’s moving faster, and therefore sweeps through a larger are of space during the same time span, than when it’s on the opposite side of its orbit, and thereby encountering more “whatever”. (Unless, of course, the “whatever” is traveling even faster than, and in the same general direction as, the Solar System. In that case, one would expect Earth’s trailing edge to hit more “whatever” when it’s moving slower.)

I have the same question as Celtic_Evolution:
“what kind of detector was used to attain the data points and what, exactly, was detected?”
Until I suss this part out, I have a gap in my info-parsing skills.

Also, a temporal POV question regarding the fact that we’re (us Solar System people) orbiting the Milky Way. If we’re spinning relative to the center of the galaxy, wouldn’t our view of the stars change? I realize that amount of space involved means a “no” answer, but any comment that helps conceptualize the dimensional aspects involved is appreciated.

No, we don’t. We can calculate it in as a plug-in to our equations (assuming our theories of gravity applied to large-scale structures are correct in the first place), but it has never been detected before. I notice that the error bars on the vertical axis are pretty darn big, too; in most cases, more than half the amplitude of the sine wave. The data on that graph certainly does not justify a claim of 8.2 sigma. One might as well argue for the existence of the Flying Spaghetti Monster based upon the data shown.

TMB says: “Viggen: Yes, it is fixed with respect to the galaxy as a whole… because it *is* the galaxy as a whole (at least, most of the mass of it). It’s not a universally special frame (ie. it would be different if we were in a different galaxy moving with respect to ours), but it is a well-defined frame with respect to us.”

This still makes no sense to me. I’m not going to argue about it because it’s not my specialty. I do know something about GR, and this feels like a manufactured argument that is in defiance of it: asserting some “whole galaxy” frame is not supportable. Most of the point of GR is that there *isn’t* some “well-defined” frame with respect to us. Gravity exists because the frame of reference is continuously distorted by the presence of mass. Unless the Italian experiment can be reproduced to eliminate some annual sun-earth phenomenon, perhaps local to the detector, I think it is a deeply questionable result.

TMB: Sometimes! I’m all over it when there’s something interesting involving Dark Matter (or, more likely, not involving dark matter) and actual scientific explanations. Here’s a nice list of coincidences that line up right around when the modulation signal peaks:

“…. If weâ??re spinning relative to the center of the galaxy, wouldnâ??t our view of the stars change? ….”

Most of the stars we can see are pretty close to us on a galactic scale and are moving around the galactic centre at about the same speed and in roughly the same direction so their positions relative to us change pretty slowly (though stars do have “proper motions” across the sky). Also, it takes us about 200 million years to go round the galaxy so any change of perspective is not likely to be blindingly obvious over the course of an evening or two.

I’m rooting for DM being something that has absolutely no physically detectable connection with this universe other than gravity. If it doesn’t turn out to be the gravitational interaction of baryon-like particles across branes, I will be sorely disappointed.

That would be great. If that could be shown, then brane theory would get a huge boost. The reason it seems like a possibility is that there are some hypotheses that state that the reason gravity is so weak compared to (and hence hard to unify with) the other forces is that it really isn’t weaker, but that much of the force “leaks” out of our universe. That would be brilliant. Of course, my knowledge of this is very limited, so perhaps Phil will come back and smash our hopes to tiny, yet massive, hard to detect bits. 😉

“Nothing implies that whatever-it-is needs to be “stationary with respect to the Milky Way”.
When the Earth is “facing into the wind”, it’s moving faster, and therefore sweeps through a larger are of space during the same time span, than when it’s on the opposite side of its orbit, and thereby encountering more “whatever”.”

Then what is the significance of a peak in June and a trough in December? Why not a peak in April and a trough in October? Why is the earth moving “into the DM wind” in June? Doesn’t it signify something about the relationship of DM to the galaxy?

Is DM uniform in distribution, or maybe in clumps. Visible matter certainly clumps (think earth, sun, Milky Way, local groups, branes). Visible matter clumps in gaseous states as well. The results from the Italians seems to indicate a uniform distribution of DM at least in the local area. To put a completely silly spin on this, consider how “dust bunnies” are formed under beds. One thought is that the scarey monsters hiding under beds roll dust bunnies together durning the day when nothing is happening and no one is around to scare. DM could very well be like the idea of monsters and dust bunnies. “We know monsters are under beds because ther are dust bunnies.” “We know there is DM because we have a cyclic variation if flashes of light.” Just a thought.

Here’s what’s happened: The Gran Sasso National Laboratory has several independent detection experiments running on site. One is a neutrino oscillation experiment which intersects a neutrino beam projected from CERN. Turns out, interestingly enough, that the DAMA detector is pretty good at registering weak interactions from that beam. CERN, in turn, modulates its beam strength with an annual periodicity, which is due to vacation schedules, which in Switzerland are strictly enforced by law.

Now you know the awful truth. The Dark Matter signal is rooted in peoples’ insatiable hunger for regular time off.

This is sooo much liek th Michaelson-Morley Experiments of a century ago to detect the Aether of Aether Theory. Only this seems to have returned a positive result.

Very Cool!!

The most important thing is that if confirmed it proves that Dark Matter exists! The only other option besides dark matter is another theory of gravity and gravity as it is is complicated and mysterious enough.

The significance of June/December vs. other months is that it is during those months that the Earth’s movement around the Sun is best aligned with the motion of the Sun around the Milky Way. If an observer on the Sun were watching the Earth as it was being lit by the radiation escaping from the accretion disk around the black hole at the center of the Milky Way, at June 2 that observer would see a “Full Earth” while at around December 2 (presumably) would be a “New Earth.”

I changed the word “interstellar” to “extraterrestrial”, Sean, since you make a valid point. It could be solar, though the amplitude of the effect seems to belie that. I was thinking “outside the Earth” as I wrote that, but put down “interstellar” since that seemed more likely. Better to be safe, I suppose.

Like the Earth, our nearest, natural satellite — the Moon — must also be encountering this Dark Matter fluctuation/field. A theory some years ago suggested that the Dark Matter field may have imparted enough energy to the mass of the Moon that it caused it to recess and lengthened our earth-day. Moreover, this DM effect, according to the research, showed that the DM field may have slowed down the acceleration rate of Mars’s rotation. While I’m not sure if this setup (Earth/DM in comparison to Moon/DM) is related, it begs the question, is there any way that we could look at the Moon ‘now’ from Earth for this DM oscillation effect?
John — http://www.moonposter.ie

Phil, my point was that this signal seems very plausibly (much more likely, I would personally bet, but who knows) to be due to some unknown systematic effect right here on Earth with an annual modulation — ski season, for example. That wouldn’t cause a change in the dark matter flux, but could cause changes in the count rate for all sorts of subtle reasons. It’s far too premature to definitely attribute the signal to something extraterrestrial — these are hard experiments, and much shaking-down (and hopefully independent confirmation) will be necessary before we understand the signal that well.

If DAMA experiment team is confident enough about the source of this monumental discovery, they should counter Juan Collar at the Cosmic Variance head on with their data and their interpretation. I bet they welcome DAMA’s response. If they don’t, that itself seems to provide a clue as to the confidence level of their
claim.

Excluding small subtle effects like the Ski season meantioned by Sean above somehow causing the modulation looks difficult. But there are a few reasons to believe that the detected effect is nontrivial. Apart from the detailed limits on background effects given by DAMA in their paper, there are a few things we can see from the data itself:

1) The amplitude and phase of the annual modulation are consistent with what you would expect from a DM signal.

2) The DAMA/LIBRA signal is within the error margin of the DAMA/NaI signal, except for a 2 sigma deviation in one energy bin. After dismantling the old apparatus, and installing the new detector and presumably making changes to other things as well, how likely is it for the alleged systematic error to cause a fake DM like signal within the bounds of the previously “detected” signal?

3) Absense of spurious signals. Sean’s hypothetical ski season effect has a period of one year, but such an effect, however it is supposed to work, will generically have higher harmonics with relatively large amplitudes. The effect causing the detected signal does not. It seems to vary quite smoothly over the course of a year, consistent with how an expected DM signal should behave.

4) Absense of an annual modulation in the multiple hit signal. The DAMA/NaI and DAMA/LIBRA experiment use not one but a few detectors. Unlike weakly interacting DM particles, cosmic rays like muons have a significant probability of hitting more than one detector. By looking at the multiple hit events you can estimate the background. If the annual modulation were to be caused by variations in the muon flux you would see an annual modulation in the multiple hit rates as well.

Another thing to think about:

The DAMA team made public a very significant effect, but there are smaller effects like a diurnal modulation (caused by the rotation of the earth) that by now should be visible but are probably not yet statistically significant. If such signals turn out to be nonexisting in the future as more data comes in, then that would falsify that the annual modulation is due to DM.

So, I guess that the DAMA team waited until they saw a diurnal modulation constent with what one would expect given the annual modulation and went ahead with announcing the annual modulation results.

1: Don’t get confused by the fact that the consensus is that we live in a “cold dark matter” (CDM) universe… although they both refer to the same aspect of dark matter (the relative velocity dispersion of nearby dark matter particles), the “cold” in CDM basically means that they’re not moving relativistically, merely at a pedestrian 200 km/s. That’s still plenty “hot” enough to keep them from clustering around the Earth, though.

2: This is known as the “Jeans mass”, and is actually the same quantity that’s used when talking about the collapse of gas clouds to form stars. In that case, it’s the individual atoms in the gas cloud that are moving about (we normally call that temperature… see point 1 above), and you need a given amount of mass to have enough gravity to overcome a given temperature.

Viggen, think of it this way: If you walk into a sprinkler, you’ll get wetter than if you walk away from it. GR doesn’t prevent water droplets from having a special direction with respect to the center of their distribution. Same thing with dark matter in a galaxy.

In regards to DAMA observed annual oscillation,
a question comes to mind in terms of the following logic:

What about the following logic:

Given the statement:

—
The Milky Way is moving at around 600 km/s
with respect to the photons of the Cosmic Microwave Background (CMB).
This has been observed by satellites
such as COBE and WMAP as a dipole contribution to the CMB,
as photons in equilibrium at the CMB frame
get blue-shifted in the direction of the motion
and red-shifted in the opposite direction.
—

Now the earth moves around the sun at 30 km/sec
The hypothesis would be that average flux would be
when earth – sun line
would be parallel
to earth – Leo line
which occurs September 1 (sun in Leo) and March 1.
with maximum flux (earth against the CMB dipole wind) on December 1
and minimum (CMB dipole wind to earth’s back) on June 1.

This is 180 degrees out of phase with your data
but perhaps that is a reflection of dark matter reality

This CMB dipole wind and earth solar rotation
are then approximately 180 degrees of phase with data expressed in:

I’ve read more skeptical assessments of the Italian experiment, and it appears that there are other, more likely, explanations for the observed signal. The whole dark matter issue is problematic. You say “we know dark matter exists,” but that isn’t true. What we know is that the current model of gravitational mechanics cannot account for the motion observed in large scale gravitational systems. There is no observational evidence otherwise of dark matter. To invoke dark matter is reminiscent of the concept of the ether. The ether was invoked to explain otherwise inexplicable observations. Ether failed as a useful concept, because the underlying thinking was flawed. It seems quite possible that the current thinking behind gravitational theory is flawed and that dark matter will go the way of ether and phlogiston. It is a disservice to science to make statements like, “we know dark matter exists.” We know no such thing.

Do not waste time trying to find Dark Matter physically.
Go to my website ‘cosmicdarkmatter.com’ and I give
a strong hint what the Dark Matter trick is all about
along with a few more hints of Physics.